there are several species of spider that can bite humans, though most do not. Black Widows and Brown Recluse are the two poisonous spiders in Missouri. In the case of black widows, only the female is poisonous, and they are very timid spiders, they will not bite unless provoked. Brown Recluse spiders (also called Violin Spiders I think), are also poisonous, but with this species, both the male and female carry the poison and these spiders prefer to live in homes (vs. outside, like many spiders). I don't know how timid or aggressive these guys are, but I do know it can be difficult to identify them since they kinda look like normal brown spiders.
See, pretty normal:
And though their bite often doesn't kill, it does do significant damage:
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